Application of deep eutectic solvents (DESs) as a green lixiviant for extraction of rare earth elements from caustic-treated monazite concentrate

Rare earth elements (REEs) with unique metallurgical properties are listed as some of the most strategically important elements in today's world. Monazite is one of the main natural minerals for REEs which is being processed using inefficient conventional hydrometallurgical approaches with extreme environmental footprints. In this paper, an attempt was made to apply deep eutectic solvents (DESs) as a green replacement for conventional solvents such as sulfuric acid and hydrochloric acid in the process of monazite leaching. After the characterization of the monazite concentrate, eleven different DESs were synthesized and employed for the leaching experiments. Results showed that each of the synthesized DES was unable to extract REEs from the phosphate phase. By sodium hydroxide roasting at 500 °C for 2 h followed by water leaching, rare earth hydroxides were formed from the monazite concentrate, and the new phase was characterized. Each of the eleven synthesized DES was tested on the roasted sample and it was revealed that 2ChCl:1PTSA, 2ChCl:1PTSA:1EG, and 1PEG-400:1PTSA have respectively the highest extraction efficiency. The DES comprised of 2ChCl:1PTSA exhibited significant capability for extraction of REEs from the roasted sample with more than 90 % recovery. Further experiments were carried out on the leaching process with 2ChCl:1PTSA for studying the effect of temperature, solid percentage, and time on the final recovery. At the optimum condition, the recoveries of the main REEs including Ce, La, and Nd reached 99.8 %, 99.9 %, and 94 %, respectively. These findings have implications for the development of more efficient and eco-friendly methods for REE recovery from monazite.